Instantaneous torque control brushless DC drives

Permanent-magnet brushless DC drives can have excellent performance in direct-drive servo applications provided suitable control techniques are implemented. This calls for a cascade control strategy in which the controller is divided into two control loops: an outer loop which generates the instantaneous torque reference signal required to track acceleration, velocity or position; and an inner loop which forces the developed instantaneous torque to follow the torque command. In most conventional control algorithms, the inner loop is designed to produce sinusoidal stator currents, thus giving rise to significant torque ripples if the motor has a back EMF waveform which differs appreciably from the ideal sinusoid. A digital torque control scheme is proposed which uses the variable structure approach to generate the on-off switching patterns (vectors) directly with a microprocessor. At every switching instant, the optimum vector is selected to force the developed torque to follow the torque reference while keeping the direct axis current as close to zero as possible for the highest efficiency. The instantaneous torque feedback signal can be estimated from the terminal quantities of the motor. Simulation results are presented